1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus which uses cholesteric liquid crystal as a display medium.
2. Description of Related Art
In recent years, reflective type liquid crystal displays are studied and developed into displays of mobile telephones and portable personal computers. Reflective type liquid crystal displays make displays by reflecting external light and therefore are advantageous of being power-saving, of being thin and light, etc.
In reflective type liquid crystal displays, generally, nematic liquid crystal is used, and these liquid crystal displays are operated typically in a TN (twisted nematic) mode and in an STN (super twisted nematic) mode. The nematic liquid crystal does not have a memory effect, and the liquid crystal displays consume power while displaying information on the liquid crystal (although the reflective type liquid crystal displays consume less power than transmitting type liquid crystal displays which use back lights).
In the meantime, lately, reflective type liquid crystal displays with a memory effect have been developed because this type of liquid crystal displays are expected to contribute to more power saving. A typical of this type is a liquid crystal display using cholestric liquid crystal. Cholestric liquid crystal is bistable, that is, has a memory effect. Specifically, cholesteric liquid crystal turns into a planar alignment state or a focal-conic alignment state depending on the voltage applied thereto and keeps in the alignment state. The cholesteric liquid crystal has the following characteristic: when the cholesteric liquid crystal is in the planar state, the liquid crystal selectively reflects light of a specified wavelength among incident light; and when the cholesteric liquid crystal is in the focal-conic state, the liquid crystal substantially does not reflect incident light. By using this characteristic, the liquid crystal display is capable of making a display without a polarizer, and the display is bright. Such a liquid crystal display using the characteristic of cholesteric liquid crystal is disclosed by SID International Symposium Digest of Technical Paper, Volume 29, 1998, page 897.
The liquid crystal display which makes a display by using the selective reflection of cholesteric liquid crystal and without using a polarizer is driven by a method in which a chain of voltage pulses is applied to the liquid crystal, the chain of voltage pulses including three steps, namely, a reset step of resetting the liquid crystal to a homeotropic state, a selection step of selecting an alignment state in which the liquid crystal will finally stay under zero-volt application, and an evolution step of causing the liquid crystal to evolve to the selected alignment state. This driving method is disclosed by International Publication No. WO98/50804 and Japanese Patent Laid-Open Publication No. 2001-228459. The driving method in which a chain of voltage pulses including the three steps is applied has the advantage that the pulse width applied in the selection step is short, resulting in high-speed driving.
When the liquid crystal display without using a polarizer is driven by the three-step driving method, from the start of the reset step to the end of the evolution step, however, the liquid crystal is substantially in a state to transmit light, and a light absorbing layer provided on the back side of the display is visible to the user. This phenomenon occurs regardless of which alignment state the liquid crystal will be finally turned into. Therefore, every time the display is updated; the screen becomes dark, that is, a blackout occurs before a new picture appears, which gives the user an unpleasant feeling.